1. Field of the Invention
This invention relates to improvements in a process for the liquefaction of coal and, more particularly, to a process for the liquefaction of coal by hydrogenating the coal in the presence of a solvent in which an iron ore consisting mainly of a hydrate of iron oxide is used as the catalyst.
2. Description of the Prior Art
It is well known that coal can be liquefied by heating a slurry composed of the coal, a solvent and a catalyst under an elevated pressure of hydrogen. The catalysts which are known to be useful for this purpose include cobalt-molybdenum catalysts, nickel-molybdenum catalysts, and halide catalysts such as zinc chloride, tin chloride, etc. However, these catalysts are disadvantageous in that their catalytic activity is greatly reduced in a short period of time and their recovery from the coal liquefaction product is very difficult. Accordingly, it is desirable to use an inexpensive and disposable catalyst in the liquefaction of coal.
Typical examples of such a disposable catalyst are iron catalysts such as iron sulfide, iron sulfate, hematite, red mud, etc. Among others, iron sulfide of the formula FeSx is known to be highly effective in enhancing the yield of liquefied coal.
In some conventional processes using iron sulfide as the catalyst, this is accomplished by utilizing the iron sulfide present in the ash of coal or by feeding iron or a compound thereof and sulfur, together with a slurry composed of coal and a solvent, to the coal liquefaction step so that iron sulfide may be formed in situ. In addition, there have been proposed a process in which ferrous waste produced abundantly from ironworks or the like is used in place of the aforesaid iron or compound thereof (Japanese Patent Laid-Open No. 105504/1978) and a process in which the sulfur fed to the coal liquefaction step is replaced by a mixture of hydrogen sulfide gas and hydrogen gas (Japanese Patent Laid-Open No. 124006/1979).
However, all of these processes have the disadvantage of using sulfur or sulfur-containing substances. More specifically, one advantage of the liquefaction of coal by hydrogenation is the so-called desulfurizing effect, that is, a reduction in the sulfur content of the coal liquefaction product. However, if sulfur or hydrogen sulfide gas is used as the catalyst, it combines with the coal liquefaction product to elevate the sulfur content of the coal liquefaction product, especially of its high-boiling fractions having a boiling point of 300.degree. C. or above. Accordingly, this coal liquefaction product can hardly be used as a clean energy source, coke, or high-quality carbon materials including a binder for electrodes, carbon fiber and the like, and graphite materials. Moreover, the increased evolution of hydrogen sulfide gas inevitably requires an enlargement of the desulfurizing equipment, thus making the coal liquefaction process expensive.
On the other hand, iron catalysts containing little or no sulfur (for example, hematite consisting mainly of anhydrous iron oxide) have the disadvantage that their catalytic activity is too low for the liquefaction of coal.